This invention relates generally to electromagnetic clutches and, in particular, to such clutches which are used to control the transmission of an automobile engine to a refrigerant compressor of an automobile air-conditioning refrigerating system.
FIG. 1 illustrates a known electromagnetic clutch which is provided between an automobile engine and a compressor to selectively drive the compressor. That clutch, as shown, comprises a pulley 1 mounted on a bearing 2 which, in turn, is mounted on a tubular extension 3 of a compressor housing 4 and is rotated by a belt 5 from the automobile engine (not shown), as disclosed, for example, in U.S. Pat. Nos. 3,044,594 and 3,082,933.
The pulley 1 is made of a magnetic material, for example, steel and comprises an outer annular cylindrical portion 1a having V-shaped grooves 6 in an outer surface thereof, an inner annular cylindrical portion 1b mounted on the bearing 2, and an axial end plate portion 1c connecting between the outer and inner annular cylindrical portions at an axial end thereof to form an annular hollow portion 7 therebetween. The axial end plate portion 1c is provided with a plurality of concentric slits 8 to form a plurality of concentric annular magnetic pole faces at an axial end thereof.
An annular electromagnetic 9 is stationarily disposed in the hollow portion 7 and is secured to the compressor housing 4 by means of bolts. The electromagnet 9 comprises an annular magnetic housing 9a having a U-shaped section and an annular electromagnetic coil 9b contained therein.
A drive shaft 10 is rotatably supported in a bearing 11 mounted in the tubular extension 3. A hub member or an annular radial flange 12 is secured on a projected end of the drive shaft 10. A magnetic armature plate 13 is supported by leaf springs 14 on and around the hub member 12 with a radial gap 15 therebetween so as to be capable of limited axial movement and to face the axial end plate portion 1c with an axial gap 16 therebetween.
A shaft seal 17 is provided around the drive shaft 10 in the tubular extension 3.
When the magnetic coil 9b is energized, magnetic flux flows through a closed loop comprising the magnetic housing 9a, the outer annular cylindrical portion 1a, the armature plate 13, the axial end plate portion 1c, the armature plate 13, and the inner annular cylindrical portion 1b, as shown by the broken line .phi.. Thus, the armature plate 13 is attracted to the axial end plate portion 1c, so that the drive shaft 10 is rotated together with the pulley 1 by the engine output. When the magnetic coil 9b is not energized, the pulley 1 is rotated by the engine, but the compressor is not driven.
In the above described clutch, the transmission of the rotating force from the pulley to the drive shaft is effected by the friction at the contacting surface between the pulley 1 and the armature plate 13. Therefore, the magnetic attraction force between the pulley and the armature plate is required to be quite large. Since the magnetic attraction force is related to the magnetic flux flowing through the closed loop, it is advantageous for the magnetic resistance of the closed loop to be relatively low.
As shown in FIG. 2, since the diameter of the inner annular cylindrical portion 1b is smaller than that of the outer annular cylindrical porton 1a, the sectional area of the inner annular cylindrical portion 1b is generally smaller than that of the outer annular cylindrical portion 1a. Therefore, the magnetic flux is restricted by the sectional area of the inner annular cylindrical portion 1b, especially at the axial extent (A in FIG. 3) between the magnetic housing 9a and the axial end plate portion 1c, because the magnetic housing 9a and an outer ring of the bearing 2 exist in parallel with the other axial extent of the inner annular cylindrical portion 1b than the axial extent A thereof to increase the sectional area of the magnetic path, as shown in FIG. 3.
The increase of the radial thickness of the inner annular cylindrical portion 1b increases the sectional area of the inner annular cylindrical portion 1b but also increases the dimension and the weight of the clutch.
In the known electromagnetic clutch, if lubricating oil leaks out through the shaft seal 17, the leaked oil may flow into the axial gap 16 between the axial end plate portion 1c and the armature plate 13. This results in the reduction of the friction between the axial end plate portion 1c and the armature plate 13.